U of T Scarborough PhD student Sergi López-Torres developed a new model to examine Darwinius, the best preserved fossil primate known to exist. (Photo by Ken Jones)

A famous fossil of an early primate shares more in common with
modern lemurs based on how its teeth erupted, according to new model
developed at U of T Scarborough.

The model, developed by PhD student Sergi
López-Torres and Associate
Professors Mary Silcox and Michael
Schillaci, re-examined the interpretation of
Darwinius, the best preserved fossil primate known to
exist.

By looking at the sequence in which adult teeth come in –
known as dental eruption – in primates, they found it had more
in common with lemurs than squirrel monkeys, the model species used by
the researchers who discovered Darwinius. Since
Darwinius died before reaching adulthood, the fossil offers
clues about the sequence in which its teeth erupted.

“Every species has a particular pattern by which their teeth
come in and this allows us to estimate the age of fossils that died
before their adult teeth could emerge,” says
López-Torres. “It seems that the pattern of dental
eruption for Darwinius is more similar to that of lemurs than
to that of monkeys.”

Before looking at Darwinius, López-Torres did a
large study of 97 living and fossil primates in order to get a clearer
picture of how different species compare through patterns of dental
development. He found that the three most primitive ancestors –
the ancestor to lemurs and lorises, the ancestor to monkeys, apes, and
tarsiers, and the ancestor to all primates – share the same
eruption sequence with each other. That pattern shares some
similarities with the dental eruption sequence found in Darwinius.

“The major difference is we found that anthropoids (ancestors
to monkeys, apes and humans) are characterized by a late eruption of
the third molar, which is something Darwinius clearly
doesn’t show,” he says. “One idea that still stands
links Darwinius to anthropoids, but since it doesn’t
show this late eruption, it looks more like a modern lemur.”

Their model also suggests Darwinius was a little older at
the time of death and would have weighed slightly less as an adult
than the original estimates predicted.

The team that originally discovered Darwinius argued the
47-million-year-old fossil was more closely related to haplorrhines,
the group that includes anthropoids (monkeys, apes, and humans) and
tarsiers. Subsequent studies by the same group suggested
Darwinius was specifically related to anthropoids, the
primate lineage in which humans belong.

Other researchers argue that Darwinius is more likely a
strepsirrhine, meaning it belongs to the opposite branch of the
primate family tree, closer to lemurs and lorises.

“Our findings don’t entirely support the strepsirrhine
hypothesis, but it’s certainly consistent with it,” says
López-Torres. “We can say for certain it’s not
consistent with the anthropoid hypothesis.”

While the new model proposes only a slight change in adult weight
and age at death – 622-642g and 1.05-1.14 years compared to
original estimates of 650-900g and 9-10 months – the findings
are significant in terms of figuring out what Darwinius was
actually like.

“It may seem trivial going from 9 or 10 months to a little
over a year, but if you consider that, for example, some species of
lemur can reproduce at a year old, this difference could mean a major
change in what the life of this animal was like,” notes
López-Torres.

“Our goal as palaeontologists is to bring these animals back
to life. It’s the best preserved fossil primate. It even has
stomach contents, so there’s a lot of potential for
understanding its biology,” adds Silcox.

“We want to be able to answer broader evolutionary questions,
but we also need to have a nuanced view of what this particular animal
was like.”

The model is explained in a new research published
online in the journal Royal Society Open Science.

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